The structure of I-Crel, a Group I intron-encoded homing endonuclease

The structure of I-Crel provides the first view of a protein encoded by a gene within an intron. This endonuclease recognizes a long DNA site ∼20 base pairs in length and facilitates the lateral transfer of that intron. The protein exhibits a DNA-binding surface consisting of four antiparallel β-strands that form a 20 Å wide groove which is over 70 Å long. The architecture of this fold is different from that of the TATA binding protein, TBP, which also contains an antiparallel β-saddle. The conserved LAGLIDADG motif, which is found in many mobile intron endonucleases, maturases and inteins, forms a novel helical interface and contributes essential residues to the active site.